1. Field of the Invention
The present invention relates to a communication control system for terminating call control information and packet information in an integrated service digital network (ISDN) after distributing the information to separate terminal unit. The present invention specifically relates to a maintenance communication control system in the ISDN service for establishing the consistency of the communication control state among terminal units being used.
2. Description of the Related Art
Recently, an integrated service digital network (ISDN) for providing various kinds of information over a digitally integrated communication network has been widely used. For example, in a basic interface of the ISDN, a user can communicate with correspondents using two information channels referred to as a B channel having a transmission speed of 64 kbits/s and one signal channel referred to as a D channel having a transmission speed of 16 kbits/s (this configuration is called "2B+D").
The B channel is used to transmit information communicated among users such as common telephone communication signals, facsimile signals, etc. The D channel is used for transmitting call control information (information about a call, a disconnection, etc.) and packet information.
The configuration of a terminal unit for terminating an ISDN service may possibly include a configuration in which all information in the above described B and D channels are terminated by one switching system.
Generally, however, line information through the B channel and the packet information through the D channel are processed in quite different manners.
Therefore, there arises a suggestion that the communication information through the above described two B channels and the call control information through the D channel are terminated by a common digital switching system (referred to as a main system). The packet information through the D channel is received by the above described digital switching system, and sent to a terminal unit used exclusively for packet information (referred to as a packet subsystem (PSS)), and then the information is terminated there. In this configuration, a workload of the communication network can be shared by two exclusive units. Accordingly, the structure of each. terminal unit can be simplified, and a conventional digital switching system can be used as a main system without large-scale addition or alteration.
In this case, the information in the D channel is transmitted in a procedure called a "link access protocol for D-channel (LAPD)" using a variable length signal frame based on a high level data link control procedure (HDLC). The HDLC is prescribed as an X.25 interface relating to a packet switching network recommended by the International Telegraph and Telephone Consultative Committee (CCITT). Additionally, the LAPD is a protocol in the layer 2 (the second layer) in the reference model of an open system interconnection (OSI), which is admitted by the CCITT. It contains an area called "an address field" and an area called "an information field."
The LAPD transmits the call control information and the packet information using the D channel in a basic interface as described above. A plurality of terminals can be accommodated in one basic interface using an in-house bus connection with a view to simultaneously perform communication at each terminal unit. To realize this function, the LAPD can establish a plurality of layer 2 links on a basic interface, thereby assuring the independence of information transmission in each link. This link access protocol is called "a multiplexed LAP".
In the LAPD, that is, a multiplexed LAP, a type of information transmission function such as the above described call control information and packet information, etc., is identified by address information called "a service access point identifier (SAPI)" stored in an address field of a frame. If SAPI=0, the frame stores call control information. If SAPI=16, it stores packet information. Likewise, address information called "a terminal endpoint identifier (TEI)" identifies one of a plurality of terminals accommodated in the same in-house system.
In the configuration of a load-sharing terminal comprising the above described main system and a PSS (packet subsystem), a basic interface is accommodated in a main system. In a digital subscriber circuit called "SPLC" in the main system, the SAPI of the D-channel frame is identified based on the above LAPD. Then, if SAPI=0 as described above, call control information is stored in the frame. Therefore, the frame is terminated by the main system, and then a call process is performed according to the information. On the other hand, if SAPI=16 as described above, packet information is stored in the frame. Therefore, the frame is transmitted from the main system to the PSS and terminated by a packet link controller called "PLCA" in the PSS, thus performing a switching operation.
However, in the configuration described above, communication control is made by distributing a workload to two terminals. Therefore, inconsistency arises in the communication control states between these two terminals, bringing a possibility that a malfunction or a fault may arise.
For example, if control is made such that a TEI is assigned by the SPLC of the main system but not on the side of the PSS with a view to enabling the establishment of a logical link for each terminal on one basic interface described above, and unless control is made for consistency of the TEI assignment between the main system and the PSS; there is a possibility that an abnormal condition arises where the PSS establishes a link, etc. when a packet terminal that is not assigned a TEI issues a TEI illegally and packet information is given as SAPI=16.
On the other hand, the control of a layer 1, that is, the electric control of a physical line, in a basic interface is performed by a digital subscriber circuit, that is, the SPLC, to which the interface is connected directly. With a view to saving electric power consumption, the subscriber circuit common unit SPLC turns the physical line to the inactive state where a signal is not sent electrically when no links are established on the physical line in the basic interface. The SPLC turns the physical line to the active state when an establishment request for at least one link is issued. Therefore, when no control is performed on the consistency of the communication control state between the main system and the PSS, the SPLC erroneously determines that all the links are terminated in the basic interface, and possibly deactivates and disconnects the physical line although a packet information link has not been terminated in the PSS.
To prevent the above described inconsistency of the communication control state, control must be performed on the consistency of the communication control state between the main system and the PSS. In the prior art technology, there are no communication control systems which efficiently control for the consistency of the communication control state without charging an excess workload to the main system and the PSS.